In discussing the background of the invention, it is useful to refer to a series of inventions covered by patents applied for by Cacace et al. These patents and the processes described therein are referred to herein as the “earlier Cacace” patents and processes. The most recent of these appears to be the family of patents that include U.S. Pat. No. 6,706,416.
The earlier Cacace patents deal essentially with the production of long products such as reinforcing bars (hereinafter referred to as “rebars”) comprising a core of mild steel and having a stainless steel cladding. These rebars are produced from billets comprised of a stainless steel jacket filled with briquettes of mild steel swarf. The billets can be heated and rolled into finished rebars having the desirable properties and low cost of mild steel but which have a stainless steel cladding for substantially increased corrosion resistance. On perusal of these patents it is clear that the achievement of a satisfactory metallurgical bond at the interface between the stainless steel cladding and the steel core has been problematical. The root of the problem is the occurrence of oxidation at elevated temperatures of the chrome in the stainless steel at the interface. There are several potential sources of the oxygen that causes this oxidation. One source is the residual oxygen in the air that remains in the briquettes and in the jacket after the billet is formed. A second source is atmospheric oxygen that enters the billet through its ends, particularly after the billet is heated. This can happen when the billet cools after it is removed from the furnace, causing the gas pressure inside the billet to drop below atmospheric pressure. It can also happen as the billet is heated due to the thermal gradient between the core and the much hotter cladding. As a result, a gap develops between the core and the cladding and this is further exacerbated by the thermal expansion of the stainless steel, which is greater than that of mild steel. A third potential source of oxygen is the residual oxidation (rust) that is present on the surface of the particles of mild steel swarf that make up the briquettes. In the absence of preventive measures, this oxidation reacts with carbon that, as the temperature increases, diffuses out of the mild steel to form CO (carbon monoxide) and/or CO2 (carbon dioxide). Both CO and CO2 can cause significant oxidation of the stainless steel at elevated temperatures.
In the process described in U.S. Pat. No. 6,706,416 this problem has been addressed by the use of dual additives which are mixed with the swarf particles before the briquettes are formed. The working examples of the first of these additives are powdered ammonium chloride (NH4Cl) and urea. When the billet is heated, these evidently break down into gaseous form at a temperature below which the oxidation of the stainless steel is significant. These gases are under pressure in the hot interior of the billet and act to displace the residual oxygen. This first step is employed in conjunction with the action of the second additive. This second additive, the working example of which is aluminium, becomes increasingly reactive as the temperature increases above that at which the ammonium chloride or urea has completely broken down. The aluminium reacts with oxygen in the rust to form aluminium oxide and also with any oxygen that enters the billet from the atmosphere, thus preventing oxidation of the chrome.
In U.S. Pat. No. 6,706,416 it is stated that “both NH4Cl and urea generate considerable volumes of reducing gases in the temperature range from 200° C. up to about 500° C.”. A similar statement appears in U.S. Pat. No. 5,676,775 in which the use of a single additive such as NH4Cl and urea is suggested. These statements are inaccurate insofar as they suggest that NH4Cl and urea generate gases that reduce Cr oxides in the billet. In fact the named agents evolve nitrogen (N2), hydrogen (H2) and chlorine (Cl2). The Ellingham diagram for the reaction of metals to form oxides indicates that these substances should not be reducing to Cr oxides in the conditions existing in the billet. The applicant now believes that it is more likely that their evolution creates a positive gas pressure in the billet. The gases are thus carried out of the billet and, in the process, drive residual air out of the billet. So, from a temperature well below 500° C., the quantity of residual atmospheric oxygen in the billet would diminish until it is probably close to zero. The remaining sources of oxygen in the billet would be the iron oxide on the surface of the swarf and air that enters through the ends of the billet after the NH4Cl and urea are spent.
As stated in U.S. Pat. No. 6,706,416, the iron oxide from the swarf combines with carbon derived from the mild steel swarf to form, first CO2 and then, at higher temperatures, CO. This process starts to take place on a significant scale at quite a low temperature, perhaps 300° C. CO2 is oxidising to Cr and, contrary to what is stated in U.S. Pat. No. 6,706,416, the Ellingham diagram shows that CO should be reducing to Cr oxides only above about 1225° C. Temperatures in the billet at the interface between the core and jacket may not always uniformly exceed this transition temperature because it is very close to the temperatures (1260-1280° C.) at which billets clad with austenitic SS normally exit the furnace. This could be due to temperature variations inside the billet or because the soaking times in the furnace are insufficient. The reducing reaction of CO may therefore not always be strong enough to bring about complete reduction, resulting in a micrographically visible layer of Cr oxides dispersed about the surface of the SS. A more concentrated, or even continuous, oxide layer would occur if the transition temperature is not reached at all, resulting in even less bonding at the interface and possibly product failure.
In U.S. Pat. No. 6,706,416, aluminium, the second metal that is added to the billet, is therefore relied on to ensure the reduction or prevention of Cr oxides as the temperature rises after the NH4Cl or urea are spent.
Having regard to the disclosures in the earlier patents, it is clear that, in the processes described therein, each reducing agent on its own is insufficient to prevent the formation of Cr oxides that impede subsequent bonding of the SS jacket to the core.
It also seems clear that, for an open ended billet comprised of granulated mild steel briquettes, as used in the earlier process, it is essential that both additives, i.e. NH4Cl or urea, and aluminium should be well dispersed through the granules. In any case, it may be concluded that, for an adequate bond between the SS jacket and the carbon steel core, it is necessary is to avoid, as far as possible, the formation of Cr oxides at the interface from the commencement of heating until the jacket becomes bonded to the core.
There are significant potential disadvantages to using swarf as a feedstock for the core in the earlier process described above.
In a full scale manufacturing operation, it may be difficult to maintain a reliable source of swarf of a particular grade in a situation in which it is necessary that the end product comply with an international standard and specification.
Furthermore, it is self-evident that costly specialised machinery, some of which is described in U.S. Pat. No. 5,088,399, is required for preparing the swarf and the billets in the earlier process. In addition, because of their furnace design, most established rolling mills cannot roll from round billets. It is not easy to envisage machinery that will be capable of producing billets that comprise compressed swarf and have a cross sectional shape that is not round. Further, the size, and especially the length, of the billets, at least those described in the earlier patents, is quite small. There are only a limited number of existing rolling mills that are able to roll billets of such short length and even fewer that can also roll from a round billet. This is partly because existing furnaces are of the pusher type designed for handling square billets. Round billets require furnaces of the walking beam type. The use of small billets is likely to result in the rolling process being inefficient because modern rolling mills are designed to roll ever-longer billets to enhance productivity. Although in principle the size and length of billets that comprise compressed swarf could be increased, and the shape changed, the technical problems involved in achieving suitable machinery for this purpose might well be insuperable.
Another problem inherent in the earlier process described above, again self evident, is that the gases evolved by the NH4Cl and urea must necessarily be vented. Apparently the billet is open-ended for this reason. This is stated in U.S. Pat. No. 5,124,214, notwithstanding that it suggests the use of a cap to enclose the ends of the billet. However, this patent is dated prior to the use of any additives as described above. Furthermore, although this patent also contains a suggestion that the tube can be sealed by applying a graphite paste to the ends of the core, this would be unworkable.
The paste would rapidly become friable and porous with the moisture in the paste rapidly being driven off. This would cause the graphite to collapse and therefore no longer form the barrier intended. Moreover, the graphite would react with the steel in the briquettes at a temperature of about 1000° C., effectively forming molten cast iron and would be completely ineffective in reducing Cr oxides.
U.S. Pat. No. 5,676,775 discloses only an open-ended billet. In U.S. Pat. No. 6,706,416, an experimental billet is disclosed which contains only aluminium as an additive. Although this billet is described as closed, it is provided at each end with a vent hole to allow gases to escape from the billet. The vent holes were welded closed after the billet was removed from the furnace. Having regard to what has been said above, the applicant believes that that these vent holes would not prevent residual atmospheric oxygen causing oxidation of Cr in the billet at lower temperatures, before the aluminium additive becomes active.
One object of the invention is to provide a billet comprising a solid steel body and a cladding composed of stainless steel, or a nickel-chrome, nickel-copper or copper-nickel alloy in which oxidation which interferes with the bond between the cladding and the steel body in the finished product is reduced, at least to the extent of providing a commercially acceptable finished product.